This invention relates to digital video technology and, more particularly, to video encoding.
Digital video is formed from a sequence of images produced by a video camera. The individual images are called video frames. To produce the illusion of motion, video frames are transmitted at 20 frames per second or higher, such that the human eye does not isolate individual frames. The eye then perceives the video images as a continuous video stream.
Transmitting video may use more bandwidth than transmitting audio. A throughput of 75 Mbits per second is common for digital video while an audio transmission might occur at only 75 Kbits per second. A 56 K baud modem transmits up to 56 K bits per second. Thus, before transmitting digital video from computer to computer, an encoding scheme is typically employed.
A number of digital video encoding standards are used today. For example, some may use temporal redundancy to encode video. Temporal redundancy is the repetition observed between consecutive video frames. Using temporal redundancy, the changes from one frame to another may be stored instead of storing each entire frame before transmission.
Many personal computer-based digital video capture systems produce noisy lines along the edges of video frames. For example, the noise may result from improper handling of closed captioning signals. Alternatively, limitations in the associated hardware devices or software drivers may produce such noise.
Before being transmitted across a telephone line or other media, the video frames are typically compressed or encoded. Like all the other pixels of the video frame, the noisy pixels are encoded. The noisy edge pixels may be difficult to encode. The noisy pixels are often random and vary significantly from frame to frame. The temporal redundancy is thus reduced, so more bits may be used to encode the noisy frames than frames without the noise. Further, when a video image is subdivided during the encoding process, the noisy lines along the edges of video frames may result in spurious frequency transform coefficients which are encoded along with the image. After decompression of the encoded noise, particularly at low bit rates, severe ringing artifacts may be visible along the noisy edges of the displayed video frame.
Thus, there is a continuing need for a mechanism for encoding video frames that have noise.